Colonies of mouse marrow cells, derived from growth in agar, will be used for syngeneic and allogeneic transfer to lethally irradiated hosts. Also, colonies exposed to 5-Bromodeoxyuridine (BUdR) will be used in similar fashion. The goal is improved marrow therapy in allogeneic hosts. Attempts to define the mechanism of BUdR-enhancement of experimental metastases will be made. Possible mechanisms under study include (a) increased arrest of cells in lungs, (b) selection of cells with high growth potential during exposure to BUdR, and (c) BUdR-induced depression of antigenicity. Tumor cell surface changes, induced by BUdR, will be studied. Surface studies include Con A agglutination, action of neuraminidase, biochemical analyses of carbohydrates, and an E. M. search for viruses budding from the surface.